Question: Gas Line Sizing

[Tidybuoy]

I'm getting ready to install my tankless water heater and my neighbor (who is a licensed plumber) will assist me making sure we get it installed properly. I have a question about gas line sizing. I currently have not yet calculated the requirements but I am wondering a couple of things in advance:

Let's say my tankless ends up requiring a 1" gas line. 1) why is the input to the water heater only 3/4"? Doesn't that defeat the 1" issue?

2) my house has a 1" gas line running under the house with 3 1-inch branches which then drop down to 3/4" near the appliances (with the exception of my "tanked" water heater which drops to 1/2".

If the branch nearest the water heater converts down to 3"4" less than 6 feet away from the new water heater location, would I need to modify the line so that it is 1" diameter all the way to the water heater and then converts to 3/4" at the last 6" of line?

I guess my confusion is based on a possible 1" requirement that will ultimately be reduced to 3/4 at the appliance itself. I'm guessing the 1" requirement means that the line can hold more quantity which will be available as the appliance needs the gas and the 3/4" input will not be overly restrictive. I'm just trying to educate myself before the actual calculations and installation.

Last question: The tankless requires a sediment trap on the gas line, near the appliance. Why don't any of my other appliances have a sediment trap (i.e., heater, stove, and original water heater)?

[Tidybuoy]

Sorry, wrong forum. I will repost in off topic. Opps.....